<style>
 .entry-form-button {
     padding: 4px 8px;
     margin-right: 1.2em;
     margin-left: 1.2em;
 }
 .entry-form-menu {
     padding:8px 10px;
     border-top:0;
     width:200px;
 }
 .entry-form input {
     padding-left: 3em;
 }
 .entry-form .fa {
     font-size: 1.2em;
     left:5px;
     top:0;
 }
</style>
<button class="btn btn-sm btn-default navbar-btn entry-form-button" data-toggle="dropdown">登陆</button>
<div class="dropdown-menu entry-form-menu">
  <form class="entry-form">
    <div class="form-group has-feedback">
      <input type="text" class="form-control input-sm" id="username" placeholder="用户名/邮箱">
      <i class="fa fa-envelope-o form-control-feedback"></i>
    </div>
    <div class="form-group has-feedback">
      <input type="password" class="form-control input-sm" id="password" placeholder="密码">
      <i class="fa fa-key form-control-feedback"></i>
    </div>
    <div class="form-group" style="margin-bottom:0;">
      <button type="button" id="entry-login" class="btn btn-sm btn-block btn-default">登陆</button>
    </div>
  </form>
</div>
